I have a kayak that I love that has developed 2 small stress cracks in the bottom of the kayak next to the scupper holes. My 1st thought was fiberglass the cracks to waterproof them and add strength so they would not reform. But from what I have read on-line plastic and fiberglass don't mix. 2nd I looked at plastic welding but I am afraid that the crack will reform.

I patched a 275 gallon water storage tank by plastic welding it. One Crack was about 2" long and the other was a puncture about the size of a dime. Then I covered the welds with a hard cure plastic epoxy. Been three years now and it never leaked. Both patches were in the lower part of the tank where there exposed to the most hydrostatic pressure. I was impressed.

So I got my kayak fixed! Bought a wood burning kit and used a lid from a plastic bucket and learned how to weld plastic. I welded up the cracks first then I embedded some wire screen mess first in the kayak hull to hopefully add some strength so this doesn't happen again. After that I put another layer of plastic over the wire mesh. Doesn't look pretty but it works. Should get me by for a while until I but a new kayak.

As a side note I found a hole in my other kayak. On the rear keel a small hole had developed from being dragged. It was an easy fix compared to the first one.